The question is incongruous. It is not valid. "God" by definition is uncreated. That's why He is God. A created god is not a god at all---it is an idol.
God is self-existent.
Do you see the incongruity of the question? The question in essence is like this: "Who created the self-existent and therefore uncreated God?
Everything that began to exist has a cause. But we Christians never claim that God began to exist. That's not what we believe. God never began to exist because He never had a beginning. He is eternal. The question can only be valid if we Christians believe and claim that God is not eternal.
Somebody who could create space and time must be someone who is greater than space and time.
A being who is greater than space and time is both timeless and boundless. He is beyond space and time. He is both infinite and eternal. Being eternal, He is self-existent. Being self-existent, He is uncaused and therefore uncreated.
